Since 1982, high precision gravity measurements have been routinely carried
out on Mt. Vesuvius. The gravity network consists of selected sites
most of them coinciding with, or very close to, leveling benchmarks to remove
the effect of the elevation changes from gravity variations. The reference
station is located in Napoli, outside the volcanic area. Since 1986,
absolute gravity measurements have been periodically made on a station
on Mt. Vesuvius, close to a permanent gravity station established in 1987,
and at the reference in Napoli. The results of the gravity measurements
since 1982 are presented and discussed. Moderate gravity changes on
short-time were generally observed. On long-term significant gravity
changes occurred and the overall fields displayed well defined patterns.
Several periods of evolution may be recognized. Gravity changes revealed
by the relative surveys have been confirmed by repeated absolute measurements,
which also confirmed the long-term stability of the reference
site. The gravity changes over the recognized periods appear correlated
with the seismic crises and with changes of the tidal parameters obtained
by continuous measurements. The absence of significant ground deformation
implies masses redistribution, essentially density changes without
significant volume changes, such as fluids migration at the depth of
the seismic foci, i.e. at a few kilometers. The fluid migration may occur
through pre-existing geological structures, as also suggested by hydrological
studies, and/or through new fractures generated by seismic activity.
This interpretation is supported by the analyses of the spatial gravity
changes overlapping the most significant and recent seismic crises.